In recent years, environmental concerns and soaring crude oil prices have reinforced preferences for energy-saving industrial products. For example, in the automotive field, hybrid vehicles each equipped with an engine and a motor (electric) as the power source have already been marketed. A similar trend has been observed in the field of work vehicles (e.g., construction vehicles and industrial vehicles) that are mostly furnished with a hydraulic drive system using a diesel engine. There have been growing numbers of electrically driven, highly efficient, energy-saving work vehicles. If, for example, the driving part of work vehicles is electrified, i.e., if their driving source is a motor, numerous energy-saving benefits are expected, such as reduced exhaust fumes, highly efficient engine drive (in the case of the engine-equipped hybrid type), enhanced efficiency in power transmission, and recovery of regenerated power. In the field of such work vehicles, growing numbers of forklifts have been electrified. So-called “battery forklifts” driven by a motor using battery power have been commercialized.
As part of engine-driven hydraulic excavators and forklifts, so-called “hybrid vehicles” equipped with a diesel engine and a motor as the driving source have recently started to be commercialized. When constructed as a hybrid work vehicle, the wheel loader is expected to offer the effect of reducing fuel consumption. Ordinary wheel loaders are work vehicles that travel by having engine power transmitted to the wheels through a torque converter and a gear transmission (T/M) while excavating and transporting gravel with a bucket part of the work device mounted at the front of the vehicle. If the traveling drive part of this type of wheel loader is electrified, it is possible to improve the efficiency of power transmission through the torque converter and gear transmission up to the level of electrical power transmission. Furthermore, since the wheel loader frequently starts and stops during traveling while working, if the traveling drive part is electrified, the power regenerated from the electric traction motor at the time of braking is expected to be recovered.
One such wheel loader operates on a series type hybrid system and includes a motor generator mechanically coupled to an engine, a first inverter controlling the motor generator, an electric traction motor mechanically coupled to the output shaft (propeller shaft) of a traveling part, a second inverter controlling the electric traction motor, and an electrical storage device electrically coupled to the first and the second inverters via a DC-DC converter. The basic workings of this system involve primarily getting the engine to drive the motor generator to generate electric power and causing the electric traction motor to generate torque with the generated power to run the vehicle while controlling the system voltage (DC bus voltage between each of the inverters) to the predetermined value by the electrical storage device and the DC-DC converter. Also, the electrical storage device of a relatively large capacity is used in this system. As such, the electrical storage device functions not only as a power source that holds the system voltage to a predetermined level as mentioned above, but also as an assist power source at the time of engine power deficiency and as an electric power sink to absorb regenerated power from the electric traction motor during regenerative braking. For example, the electrical storage device may be a large-capacity electrical double layer capacitor or a secondary battery typified by lithium ion batteries and Ni-MH batteries.
As one technique for charging and discharging control of the electrical storage device on such a hybrid work vehicle, there exists a hybrid hydraulic excavator (construction machine) that includes a motor generator control part that controls a motor generator driven by an engine, a charging and discharging control part that controls charging and discharging between a first capacitor and a second capacitor interconnected electrically, a motor connected electrically to the second capacitor, and a control device that lowers a target charge value of at least one of the first and the second capacitors before the motor starts regenerative operation (WO2010/143628).